1. Field of the Invention
The present invention relates generally to telecommunications systems and methods for verifying the authenticity of a received message, and specifically to providing a method to verify the identity of the transmitting node.
2. Background and Objects of the Present Invention
A telecommunications network is formed, at a minimum, by a transmitter and a receiver interconnected by a communications channel. The transmitter forms at least a portion of a transmitting node, and the receiver forms at least a portion of a receiving node. Information to be communicated by the transmitter to the receiver is modulated to form communications signals that are transmitted upon the communications channel to the receiving node. Thereafter, the information contained within the communications signals is recovered by the receiving node.
In a radio telecommunications network, the communications channel is formed of a radio channel defined upon a portion of the electromagnetic spectrum. However, radio channels are inherently public in nature. Therefore, a communications signal transmitted upon a radio channel can be detected by any receiving node tuned to that radio channel. Thus, an unauthorized party may be able to intercept the transmitted communications signal. Analogous security problems are also of concern in wireline telecommunications networks in the event that an unauthorized party gains access to the wireline communications channel.
Therefore, to ensure user protection and information secrecy, many telecommunications networks are now providing security measures to the users. For example, many networks require all nodes participating in a communication to provide an authentication key at the time of call setup. In addition, once the identities of the participating nodes have been verified, the authentication keys can be used to derive an encryption key that allows the nodes to encrypt the information transmitted therebetween. However, if after authentication, encryption is not switched on, one of the participating nodes could later be replaced by an unauthorized party without the other node(s) realizing that this has occurred. This has become an especially serious problem in certain countries where only weak ciphering or no ciphering at all is permitted.
Therefore, another security measure implemented by some telecommunications networks to ensure information secrecy is the inclusion of a message authentication code (MAC) along with the transmitted information. In a digital telecommunications network, where digital information signals are formed of sequences of bits, one conventional method of introducing a MAC is to add additional bits to the digital information signal. Exactly what bits to add depends upon the signal itself and a secret key known only to the involved nodes. For example, the extra bits might be computed through a hash function. However, adding bits to the digital information signal increases the complexity of the transmitted communications signal, which undesirably increases the consumption of valuable network resources.
It is, therefore, an object of the present invention to provide a message authentication code to verify the identity of the transmitting node for each transmitted digital information signal.
It is a further object of the present invention to implement a message authentication procedure without adding to the complexity of the transmitted information.
The present invention is directed to telecommunications systems and methods for implementing a message authentication code (MAC) for transmitted digital information signals. Digital information signals typically include an error detecting code, such as a Cyclic Redundancy Check (CRC) code, to ensure reliable delivery of the information. In order to verify the identity of the transmitting node, the CRC code can be modulated by a sequence known only to the participating nodes. Thus, the CRC code not only provides an error detecting function, but also serves as a message authentication code. This modified CRC code is generated by both the transmitting node and the receiving node initializing a Linear Feedback Shift Register (LFSR) state to a value generated for the current information signal corresponding to a common key known only to the participating nodes. Thereafter, the LFSR is clocked for each new digital information signal. The receiving node further defines a sliding window in which the number of failed CRC checks is counted. If the number of failed CRC checks in the sliding window surpasses a predefined threshold, an alarm at the receiving node is activated indicating that the transmitting node may be an unauthorized party. Thereafter, the receiving node can initiate a new authentication request to the transmitting node.